Conventionally, during a write to a hard disc by a transducer head, the transducer head uses a magnetic read sensor to read up-track of the writing location to assure the desired data track is targeted during the write operation. As hard disc drive areal or bit density increases, maintaining a consistent track between the read sensor and a writing pole on the transducer head becomes increasingly difficult.
Additionally, in the continuing effort to increase areal density, hard discs with arrays of patterned cells have been designed where each cell can hold a bit of data (bit patterned media (BPM)). With BPM, data may be stored in individual cells along a data track defined by the patterned bits. However, each patterned bit must be temporally synchronized with each data bit. Otherwise, data insertion may occur on an incorrect patterned bit or a patterned bit may be skipped for data writing. Therefore, one of the challenges with BPM is placing the head over the bit of interest accurately during writing. This is especially challenging due to the small size of the patterned bits (e.g. 1 nano-meter wide) and timing control requirements in a disc system rotating at high speeds (e.g. 20-50 pico-seconds per bit or patterned cell).
Conventionally, a transducer head uses a read-before-write process to maintain alignment on a data track; however, that method does not easily apply to BPM. In BPM, the distance along a data track between a write pole and a read sensor may be an order of magnitude greater than the distance between data tracks and/or a distance between bits of interest along the same data track. Further, magnetic readers used in the current state of the art detect a dynamic magnetic signal rather than digital signal (e.g. up-down, on-off, and 0-1).